The present invention pertains to a fluid mass flow controller, particularly adapted for controlling the flow of toxic or highly reactive gases used in the fabrication of semiconductor devices and the like and including a method of operation of the controller based on data which correlates controller operation with a fluid differential pressure across a flow restrictor part of the controller and the downstream fluid pressure viewed by the controller.
Some effort has been put forth to develop precision fluid mass flow controllers, particularly flow controllers for controlling the mass flow rates of fluids, such as toxic and highly reactive gases, of the type used in the fabrication of semiconductor devices, for example. In the field of semiconductor fabrication, various gases are used in etching and vapor deposition processes, which gases are toxic to humans and are also highly reactive when exposed to ambient atmospheric conditions, for example. Mass flow controllers have been developed which measure and control the flow rate of fluids of the above-mentioned type wherein the measurements are based on thermal properties of the fluids. Other fluid mass flow controllers have been developed which are based on measuring a pressure differential across a flow restrictor or orifice. The accuracy of prior art fluid mass flow controllers of the type in question here is inadequate for many applications of flow controllers.
Semiconductor fabrication processes may require the discharge of very precise quantities of fluids (primarily gases) into a process chamber. For example, flow rates ranging from as high as twenty liters per minute to as low as a few tenths of one cubic centimeter per minute (CCM) may be required. Moreover, the response time and stabilization rate of flow controllers used to control reactive gases in semiconductor fabrication may require that the controller be able to react to an xe2x80x9conxe2x80x9d signal and be stable at the required fluid flow rate within 0.5 to 1.0 seconds. The process itself may last anywhere from a few seconds to several hours and the shutoff response time of the fluid flow controller is usually required to be less than one second. The ability for thermal based fluid mass flow controllers to react and stabilize at such rates is difficult to achieve.
Another problem associated with prior art fluid mass flow controllers of the general type discussed herein pertains to the requirements to calibrate the controllers for various process fluids. Prior art fluid mass flow controllers are typically calibrated using an inert or nontoxic calibration fluid which requires the development of conversion factors or conversion data sets. Since the use of toxic or highly reactive process fluids for calibrating each controller instrument is cost prohibitive and dangerous to operating personnel, prior art mass flow controllers are typically calibrated on an inert fluid, such as nitrogen or argon, or a fluid whose properties are similar to the properties of the process fluid to be controlled by the mass flow controller. This process of using calibration fluids and conversion factors introduces errors into the operation of the mass flow controllers, is time consuming and thus expensive. The inaccuracy of prior art mass flow controllers and the expense and time required to calibrate controllers during initial setup, as well as in replacement procedures, adds substantially to the cost of many manufacturing processes, including the fabrication of semiconductor devices, to the point that certain improvements in fluid mass flow controllers have been highly desired.
Accordingly, several desiderata have been identified for fluid mass flow controllers, particularly of the type used in manufacturing processes as described above. Such desiderata include controller accuracy within a few percent of controller setpoint (at least one percent is desired), operation at elevated or below xe2x80x9cnormalxe2x80x9d temperatures and various positions or attitudes (i.e., right side up, sideways, or upside down), without loss of accuracy, such as experienced by thermal based mass flow controllers, accurate measurement and control over a wide range of flow rates, fast response time from turn-on to achieving stable flow conditions, economy of manufacture and uncomplicated modular mechanical structure to facilitate servicing the flow controller and to facilitate changing the flow controller out of the fluid flow distribution system for the manufacturing process. Other features desired in fluid mass flow controllers include no requirement to calibrate each complete controller instrument at the time of manufacture or recalibrate the instrument after servicing, the provision of a reliable easily interchanged flow restrictor or orifice part, ease of verification of the operability and accuracy of the flow controller after servicing or changeout of a flow restrictor, the ability to accurately control flow rates for a wide variety of toxic and/or reactive fluids, particularly the hundreds of fluids in gaseous form which are used in semiconductor fabrication processes, and ease of changing the controller working data for flow rates for different gases or fluids in liquid form. It is to these ends that the present invention has been developed.
The present invention provides an improved fluid mass flow controller and method of operation. In particular, an improved mass flow controller and method of operation are provided for use in connection with controlling the flow of gaseous fluids used in the manufacture of semiconductor devices and the like.
In accordance with one aspect of the present invention, a fluid mass flow controller is provided which utilizes measurements of differential pressure across a flow restrictor and the pressure downstream of the flow restrictor to provide a more accurate reading of the actual mass flow of a particular fluid at a particular temperature. Such measurements may be carried out using only two pressure sensors or transducers and over a wide range of temperatures of the fluid being measured.
The present invention further provides an improved fluid mass flow controller, particularly adapted for controlling the mass flow rate of toxic and reactive gases, including those used in the fabrication of semiconductor devices wherein the flow controller includes rapid response time to stabilize at a desired setpoint flow rate and is accurate within setpoint conditions to less than one percent error. The controller is also operable to measure mass flow rates over a wide range of such flow rates, on the order of a ratio of maximum to minimum flow rates as great as 100 to 1. The mass flow controller does not require calibration with a process fluid or with a calibration fluid and thus no conversion factors are required in the flow measurement process.
The present invention also provides an improved fluid mass flow controller which operates by measurement of fluid differential pressures across and the fluid pressure downstream of a flow restrictor and which utilizes data for the mass flow of selected fluids within a range of differential pressures and downstream pressures to which the controller will be exposed and in which the controller will be operated. The mass flow controller and flowmeter of the invention is also operable over a wide range of inlet pressures from above atmospheric pressures to vacuum conditions experienced with so-called safe delivery systems for toxic or reactive gases. Still further, the invention includes a fluid mass flow controller which is operably associated with a control system including a suitable processor circuit, such as a digital signal processor, a nonvolatile memory for storing the aforementioned data and which may receive additional sets of data when desired.
The present invention further provides a fluid mass flow control apparatus which is of mechanically uncomplicated construction, is modular in form and is particularly adapted for rapid changeout of a replaceable flow restrictor, one or more pressure transducers and a single flow control valve associated with the controller.
The present invention still further provides a flow restrictor for which data of flow versus differential pressure and downstream pressure are available as data sets for a multiplicity of fluids, particularly adapted for use with a pressure based fluid mass flow controller or flowmeter in accordance with the invention, but is adaptable for other applications and is adapted for use with toxic and reactive gases, in particular.
Still further, the present invention contemplates a method for measuring and/or controlling the mass flow rate of a fluid by measuring differential pressures across a flow restrictor and the fluid pressure downstream of the restrictor, and particularly, but not limited to operating conditions wherein the downstream pressure is below atmospheric pressure. The invention further contemplates a method of operation of a fluid mass flow controller which does not require calibration of the controller with calibration fluids but utilizes predetermined data sets for a flow restrictor part of the controller for various types of process fluids, including those which may be toxic or highly reactive.
The invention also contemplates a fluid mass flow controller including a microcontroller or processor device adapted to receive signals from two pressure sensors, a temperature sensor and command signal inputs while providing a suitable analog output signal for a control valve associated with the fluid mass flow controller. Still further, the microcontroller is operable to support RS485 communication and various network communications for receiving data from a remote site and for supporting and inputting data to a serial EEPROM. Accordingly, the invention contemplates a method of operation of a fluid mass flow controller wherein data sets characterizing a flow restrictor for different fluids may be obtained remotely via a network for rapid change in operation of the controller on various types of fluids.
Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings.